Orthogonal frequency division multiplexing (OFDM) is employed in wireless communication systems for transmission of data between network devices in wireless local area networks. Channel estimation is typically performed at receivers to account for frequency selective fading and time varying factors (e.g., delay) due to wireless transmission over a channel.
Institute of Electrical and Electronics Engineers (IEEE) 802.11ac is a wireless networking standard that is marketed under the brand name Wi-Fi® and is directed to high-throughput wireless local area networks (WLAN5). IEEE 802.11ac provides a physical layer convergence protocol (PLCP) data unit (PPDU) format that includes a preamble and data. The preamble and data are transmitted as OFDM symbols. The PPDU format 10 is shown in FIG. 1. The preamble includes (i) legacy fields applicable to IEEE 802.11a and IEEE 802.11n, and (ii) fields specific to IEEE 802.11ac. The legacy fields include a legacy short training field (L-STF), a legacy long training filed (L-LTF), and a legacy signal field (L-SIG). The fields specific to IEEE 802.11ac include a first very high throughput signal (VHT-SIG-A) field, a very high throughput short training field (VHT-STF), one or more very high throughput long training fields (VHT-LTFs), and a second very high throughput signal (VHT-SIG-B) field. The L-STF, L-LTF, L-SIG, VHT-SIG-A, VHT-STF, VHT-LTFs, and VHT-SIG-B fields have respective time durations, as shown in FIG. 1.
The legacy fields are used for backward compatibility with network devices operating according to IEEE 802.11a and/or IEEE 802.11n standards. The L-STF is used for start of packet detection, automatic gain control, initial frequency offset estimation, and initial time synchronization. The L-LTF is used for fine frequency offset estimation, time synchronization, and channel estimation. The L-SIG field is used for determining data rate and length information and time to remain “off air”.
The VHT-SIG-A field identifies: whether packets are IEEE 802.11n or IEEE 802.11ac packets; a bandwidth; a number of data streams; a guard interval; a type of coding; a modulation and coding scheme (MCS); and whether packets are beamforming packets. The VHT-STF is used to improve automatic gain control for multiple-input-multiple-output (MIMO) network devices. One or more VHT-LTFs are included in the PPDU depending on a number of data streams and/or users (network devices) in the corresponding WLAN. Each of the VHT-LTFs is a single symbol in length and has a long training sequence that is used for channel estimation. Data tones of each VHT-LTF symbol are multiplied by entries belonging to a matrix PVHTLTF to estimate the channel (e.g., a MIMO channel). The matrix PVHTLTF is defined in IEEE 802.11n and IEEE802.11ac. At a transmitter, the VHT-LTFs are modulated and tone mapped using a first column of the matrix PVHTLTF. A receiver receives the VHT-LTFs and performs channel estimation based on the matrix PVHTLTF.
The VHT-SIG-B field is a single symbol in length and provides user specific information including data length and MCS information. The VHT-SIG-B field is primarily used for multiple user (MU) applications and is typically ignored for single user (SU) applications. The VHT-SIG-B field may include a predetermined number of length bits and a predetermined number of tail bits. Depending on a bandwidth (e.g., 20 mega-hertz (MHz), 40 MHz, 80 MHz, 160 MHz or noncontiguous 160 MHz (referred to as 80+80 MHz)) allocated for transmission of the VHT-SIG-B field, the length bits and the tail bits may be repeated a predetermined number of times. FIG. 2 shows examples of the VHT-SIG-B field for the 20 MHz, 40 MHz, 80 MHz, 160 MHz and 80+80 MHz bandwidths. The symbol size of the VHT-SIG-B field changes depending on the bandwidth to include the repeated bits. The time duration associated with transmission of the VHT-SIG-B field is the same remains constant (e.g., 4 micro-seconds (μs)) for the different bandwidths.